Voltammetry is an electroanalytical method used in analytical chemistry and industrial processes. Voltammetric techniques involve the application of potential to an electrode in an analytical cell and monitoring the resulting current flowing through the cell.
Electroanalytical detectors and voltammetric cells are known in the field and have long been used for the analysis of trace elements in the laboratory. An electroanalytical cell has three electrodes: a working electrode, an auxiliary electrode, and a reference electrode. The working electrode is the electrode where the reaction of interest occurs. The working electrode is either cathodic or anodic depending on whether the reaction is a reduction or oxidation reaction. Working electrodes are sometimes mercury electrodes, but can also be made of other materials, for example inert metals or inert carbon. Auxiliary and reference electrodes are used to establish and maintain a constant potential relative to the working electrode. A current that is equal in magnitude but opposite in charge to the current of the working electrode is passed through the auxiliary electrode. The auxiliary electrode, also known as the counter electrode, prevents undesired current from passing through the reference electrode, and usually consists of a thin platinum wire, gold wire, or any other conductive material that does not affect the sample. The reference electrode is used to measure electroanalytical potential and typically consists of an electrode with a stable and well-known electrode potential, for example a calomel electrode or a silver/silver chloride electrode.
Voltammetric instruments having a working electrode that is a dropping mercury electrode (DME) are known in the art. These classical voltammetric instruments are characterized by a mercury drop formed at the end of a glass capillary tube, the tip of which is exposed to a sample solution. The unique physical properties and surface tension between mercury and the glass capillary tube provide repeatability and stability of the mercury drop, thus providing a simple method for renewing the electrode surface by forming a new drop. However, the DME also has significant drawbacks that limit its use as an automated detector for unattended field use. The primary limitations of the DME are (1) capillary clogging that necessitates frequent capillary replacement; (2) high volume batch cells that are unsuitable for flow-through operation; and (3) large amounts of mercury waste generated during electroanalytical cell operation that trigger health and environmental concerns.
Attempts have been made to improve the classical DME. Barnes et al. in U.S. Pat. No. 4,138,322 describe a degassing system based on a glass degassing chamber. Although the system allows an intense degassing process, it suffers from unsatisfactory gas/fluid separation and a complicated and ineffective electroanalytical cell design. Yarnitsky in WO 99/28,738 describes a regenerated DME that purifies and reuses mercury through contact with oxygenated water. However, despite some adaptation of the system to flow conditions, there are persistent disadvantages related to the use of a glass capillary. Dozortsev in WO 03/019133 describes an electroanalytical cell design that eliminates the glass capillary and provides online mercury purification, however, only samples that have been previously degassed can be analyzed.
Thus, there exists a need for an integrated voltammetric system in which a mercury electrode, a degassing system, and operation of the entire instrument are integrated and adapted to automatic flow operation. The present invention fulfills this need and provides further related advantages.